[Amps] What limits DC plate supply voltage for GI7B?

[Alan Ibbetson]

The datasheet for the Russian GI7B triode says the maximum anode voltage 
for "continuous wave generation" is 2.5KV, so about 5KV peak. However 
the rating is 12KV for pulse operation. Flashover would not appear to be 
an issue even with a 6KV DC supply and the Ia/Va graph still looks sane 
at 8KV despite needing well beyond -60 volts on the grid for cutoff.

Does this mean the valve can be safely operated with an anode supply 
beyond 2.5KV as long as the thermal ratings are not exceeded? Or do bad 
things happen? If there are problems please can someone explain the physics?

The reason I ask is I've been given a 3.3KV/400mA PSU, I have some GI7Bs 
on the shelf, and the cost of an 8877 or even a pair of 3-500Zs these 
days makes me shudder.

Has anyone tried 3KV or more with a GI7B?

73, Alan G3XAQ